<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        var month=prompt('请输入月份');
        month=Number(month);
        month=parseInt(month);
        switch(month){
            case 1:
            case 3:    
            case 5:    
            case 7:    
            case 8:    
            case 10:    
            case 12:
                console.log(month+'月有31天'); 
                break;   
            case 2:
                console.log(month+'月有28天');
                break;   
            case 4:
            case 6:
            case 9:
            case 11:
                console.log(month+'月有30天');
                break;   
            default:
                alert('请输入正确的月份');
        }
    </script>
</body>
</html>